There is a holographic recording method and apparatus for recording two-dimensional images, in which digital information is converted to a two-dimensional image and an object beam having been subjected to spatial optical modulation based on the two-dimensional image information is directed to a holographic recording medium in conjunction with a reference beam, thereby recording the two-dimensional image using their interference fringes.
In this case, one pixel in a spatial light modulator used for spatial optical modulation can be associated with one bit of data. However, there is a differential encoding method for associating two pixels in the spatial light modulator with one bit of data (e.g., refer to Science 265,749 (1994) by J. F. Heanue et al.).
This is intended to prevent a bit error rate (BER) from increasing due to crosstalk from neighboring places during a reproduction operation. However, this raises a problem that the image display capability of the spatial light modulator, the image transfer capability of the optical system, and the capability of reading images on a holographic recording medium are limited, unavoidably causing crosstalk to occur between pixels.
In general, when one pixel is employed to represent one bit, the amount of light of a pixel to be detected varies thereby causing an error depending on which is greater, the number of ON pixels or the number of OFF pixels, among eight pixels that include neighboring four pixels or four pixels sharing only the apexes of the pixel.
Accordingly, an error will occur if an OFF pixel having the maximum amount of detected light cannot be distinguished from an ON pixel having the minimum amount among a few ten thousands to a few millions of pixels.
On the other hand, since a predetermined pair of two pixels is coded to either ON/OFF (data of 0) or OFF/ON (data of 1) using the differential encoding method, only such a pixel as having a larger amount of light has to be detected as being ON. That is, no error will occur if the amounts of light of only two pixels are properly identified by comparison.
An encoding method has been suggested which is further improved from the differential encoding method so as to employ a larger number of pixels as a unit (pixel block) to determine a particular number of the pixels as being ON (see B. Marcus, “Modulation Codes for Holographic Recording” in Hans Coufal et al. “Holographic Data Strage,” Springer Verlag (2000) p.283).
For example, when with six pixels defined as one block, three of the pixels are coded as being ON and the other three pixels as being OFF, the pixel patterns that can be represented by one pixel block is 6C3=20, that is, four bits can be represented by six pixels.
With the ratio of the number of bits to the number of pixels being defined as the “coding rate,” the recording density is proportional to the coding rate. The differential encoding method provides a coding rate of 50%, whereas the coding with six pixels defined as one block provides a coding rate of 67%.
As such, the larger the number of pixels forming one block, the higher the coding rate becomes. However, a coding rate of 100% will never be reached (in the case of non-differential encoding) so long as the number of ON pixels within a pixel block is constant. Furthermore, from the viewpoint of reducing the consumption of the dynamic range of a recording material, the number of ON pixels within a pixel block is desirably as small as possible, and the coding rate tends to be more reduced as the ratio of the number of ON pixels to the number of pixels constituting the block (the ON pixel ratio) becomes farther apart from 50%.
As such, since an encoded pattern with a larger number of ON pixels exhibits a higher degree of photosensitivity consumption per bit, it is possible to constitute the pattern only by a combination of a less number of ON pixels when the system performance is governed by the dynamic range of a recording material. For example, when with nine pixels defined as one pixel block, zero to four ON pixels are allowed in their coding, the number of patterns that can be represented by one pixel block is 256, resulting in a coding rate of 8/9=89%, which is advantageous in the case of the number of ON pixels being fixed.
As described above, an increase in recording capacity causes a problem with the accuracy for image detection during a read operation.
This is because unlike the case of the number of ON pixels being fixed, the simultaneous presence of encoded patterns having different ON pixel ratios may cause variations in the intensity of reproduced images on the image pickup device, leading to a problem that BER increases due to crosstalk between pixels.